Recently, reflective image display elements collectively called LCOS (Liquid Crystal on Silicon) have become of great interest as image display elements for projection image display devices. Two of the characteristics of an LCOS that are advantageous for such devices are high aperture efficiency compared to transmissive image display elements, which helps achieve bright images, and high resolution for the size of the LCOS. The LCOS controls the polarization direction of the light that it reflects in order to turn itself ON and OFF in use with a polarization sensitive beam splitter (hereinafter termed a polarized beam splitter or PBS). The PBS is generally provided in the optical path between an image display element and a projection lens.
A PBS used in a projection image display device is generally formed of two isosceles right triangular prisms with their hypotenuse surfaces bonded together (see, for example, Japanese Laid-Open Patent Application 2002-040367). At least one of the optical surfaces has a polarized light separating membrane that transmits light polarized in one direction and reflects light polarized in the other direction in order to separate and/or combine light beams of different polarization based on the polarization direction of light incident onto the PBS. Hereinafter, the polarized light separating membrane and adhesive layers that bond the membrane to the optical surfaces are collectively termed an intermediate layer.
FIG. 7 shows a schematic cross-sectional view of a prior art projection image display device that includes a PBS as described above. Light L1 that is polarized in one direction (s-polarized in this example) is emitted from an illumination optical system 53, enters a PBS 51 and is reflected toward the LCOS 54 as Light L2 by an intermediate layer 52 that is inclined forty-five degrees relative to the direction of travel of the light L1. The reflected light L3 is p-polarized, enters the PBS 51, and is transmitted through the intermediate layer 52 so as to project an image onto a screen (not shown) via a projection lens 55 as light L4. In order to obtain color images using a projection image display device provided with less than three image display elements as shown in FIG. 7, a time-sharing color separating system (not shown) would be required to be used in the illumination optical system.
FIG. 7 shows the intermediate layer 52 enlarged relative to the other optical elements in order to assist in the explanation of the operation of the PBS 51. With production errors neglected, the hypotenuse surfaces of two isosceles right triangular prisms are bonded so as to be parallel to each other at the intermediate layer 52. Thus, the intermediate layer 52 has a uniform thickness C at every point.
In projection image display devices provided with LCOSs, it is important to eliminate the influence of birefringence of optical components as much as possible in order to achieve higher contrast in projected images. Furthermore, it is desirable that the body of the PBS have a larger refractive index for better polarized light separation by the polarized light separating membrane over a wide range of wavelengths. Prism glass materials that are currently in wide use and that meet these requirements often have a refractive index of 1.8 or larger.
On the other hand, adhesives having a refractive index of approximately 1.5 are in frequent use. Therefore, the prisms and the adhesive part of the intermediate layer forming the PBS are optically discontinuous. In general, an optically discontinuous surface that is oriented at an angle such that its surface normal is not aligned with an optical path, specifically in FIG. 7 an angle of forty-five degrees, causes astigmatism in an image produced from a light beam that has been transmitted through the optically discontinuous surfaces. Thus, the optically discontinuous surface 52 oriented at an angle of 45° in FIG. 7 to the transmitted light beam L4 causes astigmatism in the projected images.
As described above, one of the advantageous characteristics of LCOSs is that of high resolution. With recent advanced techniques, LCOSs can have a pixel pitch of about ten microns. With such high resolution of LCOSs, the optical system following the LCOSs, such as a projection lens, must have improved imaging performance in the projection image display device in order to fully use this high resolution. The PBS described above is conventional and the astigmatism introduced by it was not a problem with the resolution of prior art LCOSs. However, that astigmatism is not a negligible factor in reducing resolution of projected images when LCOSs have a pixel pitch of about ten microns. In order to take full advantage of recently developed high resolution LCOSs, a PBS in which astigmatism can be more favorably corrected is desired.